Field of the Invention
The present application relates to a method for diagnosing an electrical circuit and in particular, an electrical circuit for at least one component of an SCR system like a BLDC motor of a pump intended for aqueous urea solutions, an injector, a line or a tank heater for such a system.
Description of Related Art
Legislation on vehicle and heavy goods vehicle emissions stipulates, amongst other things, a reduction in the release of nitrogen oxides NOx into the atmosphere. One known way to achieve this objective is to use the SCR (Selective Catalytic Reduction) process which enables the reduction of nitrogen oxides by injection of a reducing agent, generally ammonia, into the exhaust line.
This ammonia may derive from the pyrolytic decomposition of an ammonia precursor solution, whose concentration may be the eutectic concentration. Such an ammonia precursor is generally a urea solution.
With the SCR process, the high levels of NOx produced in the engine during combustion at optimized efficiency are treated in a catalyst on exiting the engine. This treatment requires the use of the reducing agent at a precise concentration and of extreme quality. The solution is thus accurately metered and injected into the exhaust gas stream where it is hydrolysed before converting the nitrogen oxide (NOx) to nitrogen (N2) and water (H2O).
In order to do this, it is necessary to equip the vehicles with a tank containing an additive solution (generally an aqueous urea solution) and also, with a pump for conveying this solution to the exhaust pipe.
The OBD II requirements from CARB (California Air Resources Board, which is responsible for the OBDII specification) request explicitly that all components have to be diagnosed each power cycle i.e. every time the engine is switched on and off (thus also in cold conditions when the pump is not activated) and also, periodically (for instance: every 30 minutes).
In order to do that, current and/or voltage measurements can be used. However, since generally the components have (or are connected to) a rather complicated electrical circuit comprising different branches and devices, a single static measurement of the voltage and/or current in the different branches or devices will note enable to differentiate from one error to another.